great game!
however, i was stuck at this level (see http://imgur.com/gallery/ynMjjbI) for quite a while. mushrooms keep spawning out of nowhere (instead of clowning nearby other mushrooms when I used the wrong attack). and the place where victory condition is put shows 3 out of 3 tower was built, so...... (not to mention two Mr. Faker is stuck with invincibility)
also if I pause, i will resume with a ridiculous amount of mushrooms spawn all over the battle area...
this "bug" aside, I think maybe an experience based system in addition to this centralized purchase of upgrades might make the game more interesting.. but i'm sure you already contemplated about this since it is a pretty standard idea.